Polytetrafluoroethylene (PTFE) is a thermoplastic resin excellent in heat resistance, chemical resistance, weather resistance, electrical insulation properties and nontackiness, among others, and is useful in a wide range of fields of application. However, it is very high in melt viscosity, so that it has been regarded as impossible to mold the resin by applying ordinary plastics molding processes used in molding general thermoplastic resins low in melt viscosity, such as conventional extrusion molding and injection molding techniques.
Thus, the sinter molding technique has been commonly used in molding PTFE in place of general plastics molding processes. In sinter molding of a resin, the resin in a molding powder form is preformed and the preform obtained is heated to a temperature not lower than the melting point of the resin to thereby sinter the resin particles.
The method of sinter molding of PTFE includes the compression molding, ram extrusion and other techniques.
The compression molding technique generally comprises filling PTFE in a molding powder form into a mold, compressing the same, placing the preform obtained in an oven and sintering the same, followed by cooling to complete the molding process.
The ram extrusion molding technique comprises feeding PTFE in a molding powder form intermittently into a cylinder at one end thereof, pressing each feed portion into cylinder by using a piston (ram) and causing the portion to descend and be combined with the preceding pressed-into portion, and extruding the continuous body resulting from fusion by sintering, with cooling, from the other end of the cylinder.
When a PTFE powder obtained by grinding just after polymerization is used as the molding powder in such techniques of sinter molding of PTFE, void-free, compact moldings are generally obtained with ease, and the physical characteristics, such as surface smoothness, tensile strength, tensile elongation and high-voltage insulation properties, of the moldings tend to be good.
On the other hand, the PTFE powder ground just after polymerization is generally inferior in such powder characteristics as apparent density and powder flowability and, therefore, the molding powder tends to aggregate in the hopper or small-diameter cylinder of the molding machine, causing bridging or uneven filling in the mold or cylinder, hence the handleability tends to become unsatisfactory.
Further, the PTFE powder ground just after polymerization is low in apparent density and bulky per unit weight, so that it is not easy to reduce the size of the mold or cylinder. It is thus difficult to increase the productivity per mold or cylinder; this is disadvantageous.
Granulation is a conceivable means for increasing the apparent density and/or powder flowability of the PTFE powder ground just after polymerization. However, although the granules obtained are improved in apparent density and powder flowability, shearing forces have been applied thereon in the step of granulation, diminishing the physical properties of the final product moldings; another problem thus arises.
For the purpose of improving the powder flowability and apparent density, Japanese Kokai publication Hei-07-011088 discloses PTFE-based granules prepared from a filler and a PTFE powder having an average particle diameter of not larger than 20 μm as the raw materials, but it does not describe that the moldings may have good physical characteristics.
As regards the method of granulation of PTFE, Japanese Kokai publications Hei-10-259252 and Hei-10-316763, for instance, disclose the method comprising granulating, in water in the presence of a surfactant, a PTFE powder just after suspension polymerization of 99 to 99.999 mole percent of tetrafluoroethylene and 1 to 0.001 mole percent of a perfluorovinylether or a pulverization product derived therefrom, with an average particle diameter of less than 100 μm.
Thus, PTFE molding powders having good powder characteristics tend to give moldings poor in physical characteristics, while PTFE molding powders capable of giving moldings having good physical characteristics tend to have poor powder characteristics. Therefore, selection has generally been made according to which characteristics are regarded as more important in view of the intended use.
For example, in the case of general-purpose products to be mass produced, such as ball valve seats, the handleability and/or workability in the step of molding has been regarded as more important than the physical characteristics of the moldings, hence molding powders having good powder characteristics have been selected even if the physical properties of the moldings are more or less inferior. However, even such general-purpose moldings are desired to improve physical characteristics.
For improving the physical characteristics of moldings by reducing the residual impurity content, WO 96/28498 discloses a method of producing PTFE molding powders which is characterized in that crude PTFE particles obtained by suspension polymerization are pulverized in a wet condition and then washed.
For improving the flexural fatigue resistance as a physical characteristic, WO 93/16126, for instance, discloses PTFE molding powders having a heat of crystallization of 18.0 to 25.0 J/g as determined by using a differential scanning colorimeter [DSC], a specific surface area of 0.5 to 9.0 M2/g, an average particle diameter of not larger than 100 μm and a perfluoro(alkylvinylether) unit content of 0.01 to 1% by weight and capable of giving moldings having a flex life and a creep resistance with respective specified ranges. However, it has no description of such characteristics as surface roughness Ra tensile strength, tensile elongation, and powder flowability. It does not describe any method of pulverization, either.
The PTFE moldings whose physical characteristics are regarded as more important than the powder characteristics include, among others, the so-called large-sized moldings generally having a diameter not smaller than 200 mm, for instance. Such large-sized moldings are in great need from the viewpoint of economy. However, because of their large size, there is a limit to the pressure in the step of preforming. It is therefore necessary to obtain moldings having at least certain levels of physical characteristics by applying a relatively low pressure even if the powder characteristics are more or less poor.
In recent years, the range of application of PTFE has been expanded to the field of semiconductor production, for instance, so that PTFE is now used in the production of insulating seals, insulating nozzles and other high-voltage insulating materials. In semiconductor-related fields, the moldings are required, from the nature thereof, to have high-level physical characteristics, for example good dielectric breakdown strength. Further, any procedure of cutting thereof is not desirable for the prevention of product contamination but, on the other hand, they are required to show surface smoothness.